The Celestial Sphere

(The sphere should really be rotating, not Earth)

An Ancient Greek Idea• Imagine the universe to be a hollow sphere

with the Earth at the center– The Celestial Sphere

• Idea based upon how things _________ to be

–The pattern of stars in the sky ________________

• Stars ‘embedded’ on the _________–Pretend the stars are painted on a

giant ‘bubble’ around Earth• Geocentric (________________) model

appear

does not change

inside

Earth centered

Celestial Sphere: Extension of the Earth’s Coordinate System

Celestial sphere

• N/S celestial poles

•____________• Celestial equator

•____________•Declination

•____________•Right Ascension

•____________

Like a clear bowl over your head!

Latitude

Longitude

Equator

N/S Poles

The Celestial Sphere

Earth

NCP

SCP

Celestial equator

Earth’s spin axisThe celestial

sphere

SE

AESS

WS

Earth

CelestialSphere

Ecliptic

Celestialequator

Ecliptic = apparent annual path of Sun on the CS

NCP

Path of Sunduring the

year

SCP

Ecliptic (2) = _____________________________________________

North

South

Right Ascension (hours)

Celestial Equator

Ecliptic Sun’s Path

Eas

t

Declination (degrees)

Key terms

• Celestial sphere: – _________ sphere of the sky (what it looks like)

• Celestial poles joined by line that runs through the Earth’s north and south poles

(this line is, in fact, the Earth’s spin axis)

• NCP is directly above _______________– Point about which the stars appear to rotate

• Celestial equator: – ___________ of Earth’s equator onto the CS

Apparent

Earth’s North Pole

Projection

Apparent motion: Earth’s rotation

• As Earth ________, it looks as if the celestial sphere is _________ around us

• In the north, the center of the rotation is the _______________________________

• In the south, the center of the rotation is the _______________________________

• The celestial sphere appears to rotate once every _________

rotatesrotating

North Celestial Pole - Polaris

South Celestial Pole – No pole star

24 hours

Where on Earth• Definition:

–Latitude ():The angle between the Observer, thecenter of the Earth and the Earth’sequator

(Measured in degrees)

= 0o (Equator) = 90o (North pole) = -90o (South pole)

Earth

Equator

North pole

South pole

observer

zenith

= 0o)

= -90o)

= 90o)

horizon

The latitude of an observer is equal to the altitude of the visible celestial pole

A B

To NCP

zenith

horizon

Earth

B = 90o - To CE

A + B = 90o

• latitude of observer = • altitude of NCP = A

The final steps: (hold tight)

• By definition:angle A + angle B = 90o

But, angle B = 90o - , so

A + 90o - = 90

And hence,A =

The Celestial Sphere

PrecessionIn addition to its rotation and revolution, the earth’s axis also __________ (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about ____________.

precesses

26,000 years

• Polaris won’t always be the North Star

• Orion won’t always be a winter constellation

• Horoscopes are all wrong!

Because of precession: